Support structure carrying thrust load of transmission, method of manufacturing thereof and thrust needle roller bearing

ABSTRACT

A support structure carries a thrust load of a transmission which includes a torque converter having an impeller and a turbine opposite to each other with a stator therebetween. A thrust needle roller bearing having needle rollers arranged in two rows is provided at, at least one of places respectively between the stator and the impeller and between the stator and the turbine. The support structure carrying a thrust load of the transmission, a method of manufacturing the support structure and the thrust needle roller bearing are thus obtained, with an improvement in inflow and outflow of a lubricating oil, a reduction of the differential slip of the needle rollers, and an improvement in strength durability.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a support structure carrying athrust load of a transmission, a method of manufacturing the supportstructure and a thrust needle roller bearing.

[0003] 2. Description of the Background Art

[0004] A thrust needle roller bearing is constituted of needle rollers,a cage and a race, and is structured to allow the needle rollers to bein line contact with the race. Therefore, despite of a small bearingarea, this type of bearing is advantageous because of its high loadcapacity and high stiffness. The thrust needle roller bearing is thussuitable for use under hostile conditions, for example, during a driveunder conditions like scarce lubrication and high-velocity rotation, andis accordingly used as a support structure carrying a thrust load of anautomatic transmission of an automobile (e.g. support structure for acontinuously variable transmission).

[0005] A conventional thrust needle roller bearing is known fromJapanese Patent Laying-Open No. 2002-70872 according to which at leastone of the inflow and the outflow of a lubricating oil is improved orpromoted so as to increase the amount of the lubricating oil whichpasses through the bearing per unit time. This thrust needle rollerbearing is described below in connection with FIGS. 17A-17C.

[0006] As shown in FIGS. 17A-17C, this thrust needle roller bearing 50has a plurality of needle rollers 80 and two annular cages 60 and 70.These two cages 60 and 70 respectively have a plurality of windows 61and 71 having a length in the radial direction longer than that of therollers. Roller holder portions 64 and 74 formed at these windows 61 and71 hold needle rollers 80 therebetween from above and below. The radiallength la of roller holder portions 64 and 74 of two cages 60 and 70 ismade smaller than the length l of the rollers. At least one of two cages60 and 70 is bent so that at least one of the thicknesses t1 and t2 issmaller than the thickness t0 in the direction perpendicular to theradial direction as seen in FIG. 13B (hereinafter referred to asperpendicular direction). Here, thicknesses t1 and t2 refer torespective thicknesses of portions of cages 60 and 70 that are locatedrespectively outside and inside in the radial direction with respect toroller holder portions 64 and 74, and the thickness t0 refers to thethickness of roller holder portions 64 and 74 in the perpendiculardirection.

[0007] Outer plate-like portions 62 and 72 of two cages 60 and 70 arelaid on each other in the perpendicular direction while the innermostparts 67 and 77 respectively of inner plate-like portions 63 and 73 arebent in the perpendicular direction to be laid on each other. Theinnermost part 67 of inner plate-like portion 63 is caulked and therebyfixed.

[0008] In this way, on at least one of radially outer portions 62 and 72and radially inner portions 63 and 73 that are smaller in thickness thanroller holder portions 64 and 74, the inflow or outflow of thelubricating oil can be improved or promoted to increase the amount ofthe lubricating oil passing through the bearing per unit time. Moreover,since the passage of the lubricating oil is less prone to be blocked bycages 60 and 70, the lubricating oil does not stay in the bearing. Thus,any increase of the oil temperature can be avoided and the durability ofthe bearing can be improved.

[0009] It has been done in some cases that car manufactures andautomatic-transmission manufactures use an oil to which an additive isadded for energy saving. The additive-containing oil is inferior tonormal oils in lubricating ability for bearings. When such an oil isused for a conventional single-row thrust roller bearing as shown inFIGS. 17A-17C, the following problems occur.

[0010] The conventional thrust needle roller bearing 50 shown in FIGS.17A-17C is structured to allow needle rollers 80 to be in line contactwith the race and thus the raceway surface is in rolling line contactwith needle rollers 80. Accordingly, the circumferential velocity ishigher at a radially outer part of the raceway surface relative to thecircumferential velocity at the center of rotation of the bearing.Thrust needle rollers in a single row are usually used in a portioncarrying a thrust load of such a transmission as CVT (Continuouslyvariable transmission). Then, there is a difference in circumferentialvelocity between the raceway surface and the roller. A maximumdifference in circumferential velocity arises, between the difference incircumferential velocity between the raceway surface and the roller atthe inner end of the roller and the difference in circumferentialvelocity therebetween at the outer end of the roller. This difference isgreater as the roller is longer with respect to the outer diameter ofthe roller, resulting in a greater differential slip (skew of theroller). The occurrence of the differential slip causes breakage of anoil film and consequently metal-to-metal contact. Then, themetal-to-metal contact portion generates heat and thereby surface damage(smearing) as well as surface-originated peeling are likely to occur,particularly when the rotational speed is higher. In addition, it isoften seen that the lifetime becomes shorter due to the above factors.

[0011] Some thrust needle roller bearings are structured so that thecage and the race are in sliding contact with each other. In such acase, the cage blocks flow of a lubricating oil to resist the flowthereof to an area where needle rollers and the race are in rollingcontact with each other. In particular, it is necessary that the oilflows to the rolling surface of the needle rollers. If the amount of theoil is scarce, metal-to-metal contact between the needle rollers and therace occurs, possibly resulting in surface damage at an earlier stage.

[0012] Box-shaped cages 60 and 70 of the conventional thrust needleroller bearing 50 block flow of the lubricant to needle rollers 80 andthus the surface damage at an earlier stage mentioned above is likely tooccur. Further, although two cages 60 and 70 are attached to each otherand the periphery thereof is caulked, this manner of caulking couldcause two cages 60 and 70 to separate from each other.

SUMMARY OF THE INVENTION

[0013] The present invention has been made in view of theabove-discussed circumstances and an object of the present invention isto provide a support structure which carries a thrust load of atransmission, a method of manufacturing thereof and a thrust needleroller bearing, with an improvement of the inflow and outflow of alubricating oil, a reduction of the differential slip of needle rollers,and an improvement of the strength durability.

[0014] According to the present invention, a support structure carryinga thrust load of a transmission is a support structure carrying a thrustload of a transmission which has an input shaft with its rotationchanged stepwise or continuously to be transmitted to an output shaft. Athrust needle roller bearing carries a thrust load generated by rotationof the input shaft or the output shaft and the thrust needle rollerbearing has needle rollers arranged in two rows.

[0015] Regarding the support structure which carries a thrust load ofthe transmission, the thrust needle roller bearing has needle rollersarranged in two rows, so that the length in the radial direction of oneneedle roller can be shortened as compared with that of a single-rowneedle roller. Accordingly, the differential slip of the needle rollerscan be reduced and thus breakage of an oil film and metal-to-metalcontact can be avoided. Then, a reduction of the heat generation of thesupport structure as well as high-speed rotation are achieved.Consequently, any surface damage and surface-originated peeling can beprevented to improve the durability of the bearing. Moreover, as thedifferential slip can be reduced, the support structure has a low torqueand thereby a high energy efficiency can be achieved. In this way, thesupport structure carrying a thrust load of the transmission can beobtained with an improved strength durability.

[0016] Preferably, regarding the support structure which carries athrust load of the transmission, the transmission includes a torqueconverter having an impeller and a turbine opposite to each other with astator therebetween, and the thrust needle roller bearing having theneedle rollers arranged in two rows is provided at, at least one ofplaces respectively between the stator and the impeller and between thestator and the turbine.

[0017] Preferably, regarding the support structure which carries athrust load of the transmission, the transmission includes a gearmechanism constituted of a plurality of gears, and the thrust needleroller bearing having the needle rollers arranged in two rows isprovided to carry a thrust load generated by rotation of the gears.

[0018] Preferably, regarding the support structure which carries athrust load of the transmission, the support structure carrying a thrustload of the transmission is a support structure for a continuouslyvariable transmission having an input shaft with its rotation changedcontinuously to be transmitted to an output shaft.

[0019] Preferably, regarding the support structure which carries athrust load of the transmission, the width of a groove of a first pulleyprovided on the input shaft and the width of a groove of a second pulleyprovided on the output shaft are each changed to vary the diameter ofcontact of a belt, looped over the first pulley and the second pulley,with the first pulley as well as the diameter of contact of the beltwith the second pulley and thereby continuously change rotation of theinput shaft to transmit resultant rotation to output shaft.

[0020] Thus, in the continuously variable transmission having the beltlooped over the pulleys, the differential slip can be reduced. Thesupport structure has a low torque and thereby a high energy efficiencyas well as an improved strength durability can be achieved.

[0021] Preferably, regarding the support structure which carries athrust load of the transmission, the thrust needle roller bearing has aplurality of needle rollers and two annular cages. The two cagesrespectively have a plurality of pockets having a radial length slightlylarger than that of the needle rollers. Roller holder portions that areformed at the pockets respectively are structured to allow the two cagesto hold the needle rollers therebetween in the direction from above andbelow and thereby carry a thrust load of the transmission, and needlerollers are arranged in two rows in each of the pockets.

[0022] In this way, the transmission having the double-row thrust needleroller bearing can be implemented with a simple structure.

[0023] Preferably, the needle rollers of the support structure carryinga thrust load of the transmission each have an end-surface shaperepresented by symbol A, symbol F or a combination of those representedby symbols A and F defined by JIS.

[0024] Thus, the shape of the end surface of the needle roller canappropriately selected.

[0025] Preferably, regarding the above-described support structurecarrying a thrust load of the transmission, one of the double-row needlerollers that is located on the radially outer side has its length whichis at least the length of the needle roller located on the radiallyinner side.

[0026] The radially outer needle roller thus has its length larger thanthat of the radially inner needle roller so that the load capacity onthe radially outer side can appropriately be increased to a desired loadcapacity.

[0027] Preferably, regarding the support structure carrying a thrustload of the transmission, the roller holder portion has its cornersmoothly rounded.

[0028] Thus, the needle rollers can stably be guided and held withoutbreakage of a lubricating-oil film formed on the surface of the needlerollers.

[0029] Preferably, regarding the support structure carrying a thrustload of the transmission, one of the two cages is caulked to be fixed tothe other cage on at least one of the radially outermost end and theradially innermost end of the two cages.

[0030] In this way, the two cages can surely be fixed by the caulking sothat separation of needle rollers from the cages, due to separation ofthe two cages, can be prevented. Here, the portion to be caulked mayappropriately be selected from one of the radially outer side andradially inner side of the cages and both of the radially outer andinner sides.

[0031] Preferably, regarding the support structure carrying a thrustload of the transmission, the two cages are caulked on both of theradially outermost end and the radially innermost end of the two cages,and a flat portion having a cross-sectional height lower than that ofthe roller holder portions is provided between a caulked portionproduced by the caulking and respective ends of the needle rollers.

[0032] In this way, the radially inner part and the radially outer partof the two cages are caulked, the flat portion having thecross-sectional height lower than that of the roller holder portions isprovided between the caulked portion and respective ends of the needlerollers, and accordingly not only the outflow of the lubricating oil butthe inflow of the lubricating oil can be improved. Then, seizure of anypart of the bearing can surely be prevented and drilling wear of an endsurface of a needle roller and a pocket of the cage can be reduced.Further, as it hardly occurs that the cages block passage of thelubricating oil, the lubricating oil is unlikely to stay so that anyincrease of the oil temperature can be reduced. Accordingly, togetherwith an increase in strength of the cages that is achieved by fixingboth of the radially outermost end and the radially innermost end of thetwo cages, a further improvement of the durability of the bearing can beachieved.

[0033] Preferably, regarding the support structure carrying a thrustload of the transmission, one of the two cages is bent into C-shape tobe caulked to the other cage on at least one of the radially outermostend and the radially innermost end of the two cages.

[0034] This caulking can be done to prevent the two cages fromseparating from each other.

[0035] Preferably, regarding the support structure carrying a thrustload of the transmission, the two cages are welded on at least one ofthe radially outermost end and the radially innermost end of the twocages.

[0036] This welding can be done to surely fix the two cages and furtherprevent deformation of the cages in assembly.

[0037] Preferably, regarding the support structure carrying a thrustload of the transmission, the two cages are laid on each other to be incontact with each other at a portion where they contact an end surfaceof the needle roller.

[0038] Thus, drilling wear of the pockets of the cages due to the endsurfaces of the needle rollers can be reduced.

[0039] Preferably, regarding the support structure carrying a thrustload of the transmission, the two cages are caulked to be fixed on bothof the radially outermost end and the radially innermost end of thecages, one of the two cages is bent for caulking on the outermost endand the other of the two cages is bent for caulking on the innermostend.

[0040] Preferably, regarding the support structure carrying a thrustload of the transmission, the two cages are caulked to be fixed on bothof the radially outermost end and the radially innermost end of thecages, and one of the two cages is bent for caulking on both of theoutermost end and the innermost end.

[0041] The bent portions for caulking may be bent in the same directionor in different directions on both of the outermost end and theinnermost end.

[0042] Preferably, regarding the support structure carrying a thrustload of the transmission, the two cages are caulked along the entirecircumferential periphery of the cages.

[0043] In this way, the two cages can firmly be integrated into oneunit.

[0044] Preferably, regarding the support structure carrying a thrustload of the transmission, the two cages are partially caulked at aplurality of portions in the circumferential direction.

[0045] Thus, as compared with the caulking of the two cages along theentire periphery, the caulking process can be simplified and deformationof the cages in the caulking process can be reduced. Preferably, thecaulked portions on the radially outermost end and the radiallyinnermost end of the two cages may be placed at regular intervals in thecircumferential direction with a phase difference between the caulkedportions, so that the deformation of the cages can further be reduced.

[0046] Preferably, regarding the support structure carrying a thrustload of the transmission, the two cages each have an alignment part forpositioning one of the two cages with respect to the other cage and forpreventing displacement of the other cage from the one cage.

[0047] The positioning of the two cages in assembly can thus befacilitated and displacement of one of the two cages from the otherthereof after assembly can be prevented.

[0048] The thrust needle roller bearing of the present invention is usedfor a support structure carrying a thrust load of the above-describedtransmission.

[0049] According to one method of manufacturing a support structurecarrying a thrust load of a transmission in accordance with the presentinvention, two cages and needle rollers are assembled so that the cageshold needle rollers therebetween in the direction from above and belowby roller holder portions formed respectively at a plurality of pockets,one of the two cages is caulked to the other cage, and thereafter thetwo cages and the needle rollers are carburized, quenched and tempered.The needle rollers may be quenched and tempered in advance.

[0050] The above-described method does not require annealing of thecaulked portion so that the cages having a high strength (high hardness)and a large depth of a hardened layer can be produced at a low cost.

[0051] According to another method of manufacturing a support structurecarrying a thrust load of a transmission in accordance with the presentinvention, two cages and needle rollers are each quenched and tempered,thereafter they are assembled so that the cages hold the needle rollerstherebetween in the direction from above and below by roller holderportions formed respectively at a plurality of pockets, and one of thetwo cages is caulked to the other cage.

[0052] According to the above-described method, different heattreatments can be performed respectively on the two cages and needlerollers. For example, each of the two cages can be carburized, quenched,tempered, and soft-nitrided.

[0053] Preferably, according to the one and another methods ofmanufacturing the support structure carrying a thrust load of thetransmission, the two cages are each produced by shaving or pressworking.

[0054] Thus the two cages each can appropriately be formed by shaving orpress working.

[0055] The foregoing and other objects, features, aspects and advantagesof the present invention will become more apparent from the followingdetailed description of the present invention when taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0056]FIG. 1 is a schematic cross-sectional view showing a supportstructure which carries a thrust load of a transmission according to afirst embodiment of the present invention.

[0057]FIG. 2A is a plan view showing a thrust needle roller bearingwhich is a support structure carrying a thrust load of the transmissionaccording to the first embodiment of the present invention, FIG. 2B is across-sectional view along line IIB-IIB in FIG. 2A, FIG. 2C is anenlarged view of an essential part shown in FIG. 2B, FIG. 2D is anenlarged view of an essential part shown in FIG. 2A, and FIG. 2E is anenlarged cross-sectional view along line IIE-IIE in FIG. 2C.

[0058]FIG. 3 is a partially cross-sectional perspective view showing, inan enlarged form, a part of the thrust needle roller bearing in FIGS.2A-2E.

[0059]FIG. 4 is an enlarged view of the thrust needle roller bearing andtherearound shown in FIG. 1.

[0060]FIG. 5 shows a procedure for manufacturing the thrust needleroller bearing shown in FIGS. 2A-2E, 3 and 4.

[0061]FIG. 6 shows another procedure for manufacturing the thrust needleroller bearing shown in FIGS. 2A-2E, 3 and 4.

[0062]FIG. 7A is a plan view showing a support structure carrying athrust load of a transmission according to a second embodiment of thepresent invention, FIG. 7B is a cross-sectional view along lineVIIB-O-VIIB in FIG. 7A, FIG. 7C is a bottom view of FIG. 7A, FIG. 7D isan enlarged view of a portion A in FIG. 7B, and FIG. 7E is an enlargedview of a portion B in FIG. 7B.

[0063]FIG. 8A is a plan view showing a support structure carrying athrust load of a transmission according to a third embodiment of thepresent invention, FIG. 8B is a cross-sectional view along lineVIIIB-O-VIIIB in FIG. 8A, FIG. 8C is an enlarged view of a portion C inFIG. 8B, and FIG. 8D is an enlarged view of a portion D in FIG. 8B.

[0064]FIG. 9A is a plan view showing a support structure carrying athrust load of a transmission according to a fourth embodiment of thepresent invention, FIG. 9B is a cross-sectional view along lineIXB-O-IXB in FIG. 9A and FIG. 9C is an enlarged view of a portion E inFIG. 9B.

[0065]FIG. 10 is a schematic cross-sectional view showing a supportstructure which carries a thrust load of a gear mechanism of atransmission according to a fifth embodiment of the present invention.

[0066]FIG. 11 is a schematic cross-sectional view showing a region P inFIG. 10 in an enlarged form.

[0067]FIGS. 12A and 12B illustrate respective shapes of end surfaces ofneedle rollers.

[0068]FIG. 13 is a schematic cross-sectional view showing a supportstructure for a continuously variable transmission according to a sixthembodiment of the present invention.

[0069]FIG. 14 is a cross-sectional view showing a portion P in FIG. 13in an enlarged form.

[0070]FIG. 15 is an enlarged view of a thrust needle roller bearing andtherearound shown in FIGS. 13 and 14.

[0071] FIGS. 16A-16E are each a cross-sectional view showing a shape ofa part which is bent for caulking on the outermost end or the innermostend of cages.

[0072]FIG. 17A is a partial plan view showing a conventional thrustneedle roller bearing, FIG. 17B is a cross-sectional view along lineXVIIB-XVIIB in FIG. 17A and FIG. 17C is a cross-sectional view alongline XVIIC-XVIIC in FIG. 17B.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0073] Embodiments of the present invention are hereinafter described indetail in connection with the drawings.

First Embodiment

[0074] Referring to FIG. 1, a transmission, for example, an automatictransmission is shown. The automatic transmission is usually constitutedof a torque converter 100 and a planetary gear mechanism (not shown).

[0075] Torque converter 100 chiefly includes an impeller 101, a stator102 and a turbine 103. According to this embodiment, a support structurecarrying a thrust load of the transmission corresponds to thrust needleroller bearings 1 attached, for example, between impeller 101 and stator102 and between stator 102 and turbine 103.

[0076] In this torque converter 100, impeller 101 coupled to an outputshaft of an engine and turbine 103 coupled to an input shaft of thetransmission are arranged opposite to each other. Stator 102 isattached, via a one-way clutch 104, to a stator shaft fixed to a casing.This stator 102 serves to redirect a fluid circulating between impellerblades 101 a and turbine blades 103 a that are cup-shaped, when thefluid returns from turbine 103 to impeller 101 in the radially centralportion, so as to apply a forward-direction rotational force and therebyamplify a transmission torque.

[0077] Thrust needle roller bearing 1 between impeller 101 and stator102 has needle rollers 2, two cages 3 and 4 and races 105 a and 105 b.Race 105 a is attached to an impeller hub 101 b and race 105 b isattached to stator 102.

[0078] Thrust needle roller bearing 1 between stator 102 and turbine 103has needle rollers 2, two cages 3 and 4 and races 106 a and 106 b. Race106 a is attached to a turbine hub 103 b and race 106 b is attached tostator 102.

[0079] Thrust needle roller bearings 1 attached between impeller 101 andstator 102 and between stator 102 and turbine 103 each have needlerollers 2 including needle rollers 2 a and 2 b arranged in two rows.

[0080] This thrust needle roller bearing 1 has its structure asspecifically described below.

[0081] Referring to FIGS. 2A-2E and FIG. 3, thrust needle roller bearing1 has a plurality of needle rollers 2 and two annular cages 3 and 4 thathold these needle rollers 2 at predetermined pitches in thecircumferential direction. Two cages 3 and 4 respectively have aplurality of rectangular pockets 5 and 6 with the length in the radialdirection larger than the length L of needle rollers 2, and are formedby pressing of a steel plate which is for example a cold-rolled steelplate (SPCC). Roller holder portions 5 a and 6 a are formed respectivelyon opposite sides of pockets 5 and 6 to extend toward and face eachother. Needle rollers 2 are thus held by roller holder portions 5 a and6 a therebetween in the direction from above and below. Instead of cages3 and 4 as described above, any cages produced through draw-forming of aband steel by means of a press or through shaving may be used.

[0082] Needle rollers 2 include needle rollers 2 a on the radially outerside and needle rollers 2 b on the radially inner side, and are arrangedin two rows in pockets 5 and 6. The double-row arrangement reduces adifference in circumferential velocity of revolution between theradially outer part and the radially inner part, and thereby thedifferential slip between the rollers and the raceway surface (notshown) is reduced. Then, less heat is generated from the contact portionand the surface damage (smearing) can be prevented.

[0083] Here, respective lengths L1 and L2 of double-row needle rollers 2a and 2 b are equal to each other. These lengths, however, may beselected to satisfy a relation, L2 (length of the radially innerroller)≦L1 (length of the radially outer roller) or L1≦L2 depending onconditions in use. Preferably, the length of radially outer needleroller 2 a is longer, for example, 1.2 times as long as the length ofradially inner needle roller 2 b so that the load capacity on theradially outer part is increased.

[0084] As shown in FIG. 2D, the length La in the radial direction ofroller holder portions 5 a and 6 a can be made smaller than the length Lof needle rollers so that a lubricating oil can readily pass betweenneedle rollers 2 and cages 3 and 4 through depressions 5 b and 6 bformed on both ends of roller holder portions 5 a and 6 a.

[0085] Further, as shown in FIG. 2E, respective corners (F) of rollerholder portions 5 a and 6 a may be rounded when pockets 5 and 6 arepunched by a press, or may be rounded to become smooth throughsurface-pressing of the inner rim after the punching. In this way,without breaking a lubricating-oil film formed on the surfaces of needlerollers 2 a and 2 b, needle rollers 2 a and 2 b can stably be guided andheld. Here, the corners are “rounded” so that the sharp edges of thecorners are removed to become round.

[0086] Of two cages 3 and 4, upper cage 3 has its roller holder portion5 a with its radially outer portion including a sloping extension 3 aformed by bending the outer end of roller holder portion 5 a and anouter plate-like portion 3 b formed by bending the lower end of slopingextension 3 a in the radial direction. Roller holder portion 5 a of cage3 also has its radially inner portion including a sloping extension 3 cformed by bending the inner end of roller holder portion 5 a and aninner plate-like portion 3 d formed by bending the lower end of slopingextension 3 c in the radial direction.

[0087] Lower cage 4 with the pockets punched with the same die as thatfor upper cage 3 has roller holder portion 6 a with its radially outerportion including a sloping extension 4 a formed by bending the outerend of roller holder portion 6 a and an outer plate-like portion 4 bformed by bending the outer end of sloping extension 4 a. Roller holderportion 6 a of lower cage 4 has its radially inner portion including asloping extension 4 c formed by bending the inner end of roller holderportion 6 a and an inner plate-like portion 4 d formed by bending theinner end of sloping extension 4 c.

[0088] Respective outer plate-like portions 3 b and 4 b of two cages 3and 4 are laid on each other so that these plates are in contact witheach other in the direction perpendicular to the radial direction asseen in FIG. 2C (herein referred to as perpendicular direction). Theoutermost part of outer plate-like portion 4 b is bent upward intoC-shape so that outer plate-like portion 4 b is caulked to outerplate-like portion 3 b to form a caulked portion 7. Inner plate-likeportions 3 d and 4 d are also laid on each other so that these platesare in contact with each other in the perpendicular direction. Theinnermost part of inner plate-like portion 3 d is bent downward intoC-shape so that inner plate-like portion 3 d is caulked to innerplate-like portion 4 d to form a caulked portion 8. Caulked portions 7and 8 are provided along the entire outermost and innermost edges of twocages 3 and 4.

[0089] Two cages 3 and 4 are thus firmly integrated into a unit bycaulking of the outermost and innermost edges with caulked portions 7and 8. Therefore, two cages 3 and 4 are never separated from each otherduring a drive. Moreover, a large area of contact between the endsurfaces of needle rollers 2 and pockets 5 and 6 is secured in regionsof outer plate-like portions 3 b and 4 b and inner plate-like portions 3d and 4 d so that drilling wear can be reduced.

[0090] When two cages 3 and 4 are fixed, a thickness T₁, in theperpendicular direction, of the radially outer part of the cages and athickness T₂, in the perpendicular direction, of the radially inner partthereof are smaller than a thickness T₀ in the perpendicular directionof roller holder portions 5 a and 6 a of two cages 3 and 4, because ofthe presence of sloping extensions 3 a, 4 a, 3 c and 4 c. In otherwords, between caulked portions 7 and 8 and corresponding ends of needlerollers 2, there are provided flat portions (outer plate-like portions 3b and 4 b and inner plate-like portions 3 d and 4 d) having across-sectional height smaller than that of roller holder portions 5 aand 6 a.

[0091] As shown in FIG. 4 for example, thrust needle roller bearing 1structured as discussed above is clearance fit by using caulked portion8 of upper cage 3 as a guide surface, so that needle rollers 2 rollbetween a raceway surface 9 a of race 106 b attached to the stator and araceway surface 10 a of race 106 a attached to the turbine (or theraceway surface of the race attached to the impeller and the racewaysurface of the race attached to the stator). As race 106 a rotates,cages 3 and 4 accordingly rotate with this race 106 a so that needlerollers 2 roll between raceway surface 10 a of race 106 a and racewaysurface 9 a of race 106 b. Here, a lubricating oil is supplied from ahydraulic source (not shown) through an oil path into thrust needleroller bearing 1.

[0092] The lubricating oil is supplied from the radially inner side ofthrust needle roller bearing 1 as indicated by arrow b, thereafterpassed through a space formed by the outer surfaces of needle rollers 2and cages 3 and 4 as indicated by arrow c, lubricates the portionsbetween the side surfaces of needle rollers 2 and roller holder portions5 a and 6 a of cages 3 and 4, between end surfaces of needle rollers 2and cages 3 and 4, and between the side surfaces of needle rollers 2 andraceway surfaces 9 a and 10 a, and is then discharged as indicated byarrow d through the portion between raceway surface 10 a and theradially outer portion of cage 4 with respect to roller holder portion 6a and through the portion between raceway surface 9 a and the radiallyouter portion of cage 3 with respect to roller holder portion 5 a.

[0093] Regarding the lubrication of the above-described portions by thelubricating oil, since the perpendicular thicknesses T₁ and T₂ of theradially outer and inner portions of roller holder portions 5 a and 6 aof two cages 3 and 4 are made smaller than the perpendicular thicknessT₀ of roller holder portions 5 a and 6 a (see FIG. 2C) and accordinglythe cross-sectional area of the space formed between raceway surface 10a of race 106 a and the radially inner portion of lower cage 4 withrespect to roller holder portion 6 a is larger than conventional ones,not only the outflow of the lubricating oil but also the inflow of thelubricating oil is enhanced. Seizure of any part of the bearing cansurely be prevented and drilling wear of the end surfaces of needlerollers 2 and pockets 5 and 6 of cages 3 and 4 can be reduced. Moreover,since the lubricating oil is less prone to be hindered from passing bycages 3 and 4, the lubricating oil hardly stays. Then, any increase ofthe oil temperature can be reduced, the strength of the cages isincreased and the durability of the bearing can further be improved.

[0094] A procedure for manufacturing the above-described thrust needleroller bearing 1 is detailed below.

[0095]FIG. 5 shows a procedure for manufacturing the thrust needleroller bearing according to the first embodiment of the presentinvention. Referring to FIG. 5, two cages 3 and 4 and needle rollers 2that have been quenched and tempered or have not been quenched are set.Specifically, the cages and needle rollers are assembled in such amanner that roller holder portions 5 a and 6 a formed respectively atpockets 5 and 6 of two cages 3 and 4 hold needle rollers 2 therebetweenin the direction from above and below (step S1 a). Then, the outermostpart of outer plate-like portion 4 b is bent upward into C-shape to formcaulked portion 7 while the innermost part of inner plate-like portion 3d is bent downward into C-shape to form caulked portion 8, andaccordingly two cages 3 and 4 are integrated into a unit and fixed (stepS2 a). Here, needle rollers 2 are made of, for example, a SUJ bearingsteel which is a high carbon chromium bearing steel of type 1 or type 2,and the steel is oil-quenched at a temperature of 840° C. for 30 minutesand then tempered at a temperature of 180° C. for 90 minutes to adjustthe surface hardness to approximately 700-750 by Vickers hardness (Hv).

[0096] After this, with needle rollers 2 and two cages 3 and 4 being setas described above, they are heat-treated (e.g. carburized, quenched andtempered or quenched and tempered after carbonitrided) (step S3 a). Athrust needle roller bearing is thus produced. In this case, thecarburization is done, for example, by carburization for example at atemperature of 850° C. for 35 minutes (in RX gas atmosphere), quenchingin oil, and then tempering at a temperature of 165° C. for 60 minutes.The carbonitriding is done, for example, by carbonitriding in acarbonitriding atmosphere (1-3% by volume of ammonia is added to the RXgas) at a temperature of 840-850° C. for 35 minutes and subsequent quickcooling in oil or gas.

[0097] Here, two cages 3 and 4 may be improved in strength by beingsoft-nitrided in advance at a temperature of 570-580° C. for 35 minutes.No preliminary heat-treatment may be performed on needle rollers 2.However, if needle rollers 2 are subjected to heat treatment or throughquenching in advance before being assembled, an advantage is obtainedthat the strength is further improved by the subsequent carburizing orcarbonitriding while a disadvantage of an increase of the manufacturingsteps is accompanied. At any rate, as compared with conventionallyemployed processes in which two cages 3 and 4 and needle rollers 2 areseparately heat-treated and the caulked portions are annealed, themanufacturing process is simplified.

[0098] If the soft-nitriding of cages 3 and 4 is done, the process shownin FIG. 6 may be used. Namely, cages 3 and 4 and needle rollers 2 areseparately heat-treated (e.g. carburized, quenched and tempered orcarbonitrided and then quenched and tempered) (step S1 b), cages 3 and 4and needle rollers 2 are set (step S2 b) and thereafter cages 3 and 4are caulked (step S3 b).

[0099] Thrust needle roller bearing 1 which is manufactured through theabove-describe procedure can offer specific characteristics that aredescribed in detail below.

[0100] With regard to needle rollers 2, the surface layer thereof is acarburized or carbonitrided layer and thus the hardness of the surfacelayer is higher than that of conventional ones. Therefore, even ifhigh-hardness foreign matters are caught in rollers, indentations arehardly made, which contributes to extension of the lifetime. In thecarbonitriding process, a nitrogen-rich layer is formed and such a largeamount of retained austenite as 20% by volume can be obtained. Thisfeature contributes to an increase of the hardness as well as anextension of the lifetime since the large amount of retained austeniteplastically deforms to alleviate stress concentration, which occurs inconventional bearings when a high-hardness foreign matter is caughtbetween raceway surfaces 9 a and 10 a that leaves an indentationresulting in a source of stress concentration around the indentation.Here, the nitrogen-rich layer may specifically be at least 0.1 mm inthickness and at least 750 Hv in surface hardness. Moreover, theinternal hardness can be increased to be almost equal to the surfacehardness so that the strength of the entire needle rollers can beimproved. Accordingly, when used under harsh conditions, for example,heavy load, the product of the present invention can sufficiently endurethe load and satisfy a desired lifetime.

[0101] With regard to cages 3 and 4, as needle rollers 2, the surfacelayer of the cages is a carburized layer or carbonitrided layer, and asurface hardness of at least 400 Hv, or at least 600 Hv can be achieveddepending on conditions of heat-treatment. The wear resistance can thusbe enhanced as compared with conventional products.

Second Embodiment

[0102] This embodiment has its structure different from that of thefirst embodiment only in the shape of cages and caulking method. It isnoted that the same or similar components or parts to those of the firstembodiment are denoted by like reference characters and descriptionthereof is not repeated here.

[0103] Referring to FIGS. 7A-7E, thrust needle roller bearing 11 appliedto a support structure which carries a thrust load of the transmissionhas a plurality of needle rollers 2 and two annular cages 13 and 14 thathold these needle rollers 2 at predetermined pitches in thecircumferential direction. As shown in FIG. 7D, the upper one of twocages 13 and 14, namely upper cage 13, includes a roller holder portion15 a having its radially outer portion which includes a slopingextension 13 a formed by bending the outer end of roller holder portion15 a and an outer plate-like portion 13 b formed by bending the lowerend of sloping extension 13 a in the radial direction.

[0104] Roller holder portion 15 a of upper cage 13 also has its radiallyinner portion which includes a sloping extension 13 c formed by bendingthe inner end of roller holder portion 15 a and an inner plate-likeportion 13 d formed by bending the lower end of sloping extension 13 cin the radial direction.

[0105] Lower cage 14 with pockets punched with the same die as that forupper cage 13 has roller holder portion 16 a with its radially outerportion including a sloping extension 14 a formed by bending the outerend of roller holder portion 16 a and an outer plate-like portion 14 bformed by bending the outer end of sloping extension 14 a in the radialdirection. Roller holder portion 16 a of lower cage 14 has its radiallyinner portion including a sloping extension 14 c formed by bending theinner end of roller holder portion 16 a and an inner plate-like portion14 d formed by bending the inner end of sloping extension 4 c in theradial direction.

[0106] As shown in FIG. 7D, respective outer plate-like portions 13 band 14 b of two cages 13 and 14 are laid on each other so that theseplates are in contact with each other in the direction perpendicular tothe radial direction (herein perpendicular direction). The outermostpart of outer plate-like portion 14 b is partially bent upward intoC-shape so that outer plate-like portion 14 b is caulked partially toouter plate-like portion 13 b to form partially caulked portions 17.Inner plate-like portions 13 d and 14 d are also laid on each other sothat these plates are in contact with each other in the perpendiculardirection, and the innermost part of inner plate-like portion 13 d isbent downward.

[0107] Further, as shown in FIG. 7E, outer plate-like portions 13 b and14 b are laid on each other so that they are in contact with each otherin the perpendicular direction and the outermost part of outerplate-like portion 14 b is bent upward. Inner plate-like portions 13 dand 14 d are laid on each other so that they are in contact with eachother in the perpendicular direction and the innermost part of innerplate-like portion 13 d is partially bent downward into C-shape so thatinner plate-like portion 13 d is partially caulked to inner plate-likeportion 14 d to form partially caulked portions 18. With these partiallycaulked portions 17 and 18, the inner and outer edges of two cages 13and 14 are firmly integrated and, in this case, the manufacturingprocess can remarkably be simplified as compared with the caulking ofthe entire periphery of the thrust needle roller bearing (FIGS. 2A-2E)of the first embodiment described above.

[0108] An alignment part 19 for alignment between two cages 13 and 14includes, for example, a notch (not shown) made in the outer rim ofupper cage 13 and a projection (not shown) formed on the outer rim oflower cage 14, and the projection and the notch are engaged with eachother so as to prevent displacement of pitches of pockets 15 and 16 ofcages 13 and 14 in the caulking process for example. Those partiallycaulked portions 17 and 18 are arranged at four positions on theperiphery with a phase difference of 45° between the caulked portions.In this way, deformation of the cages in the caulking process isreduced. Here, the number of positions where the caulked portions areplaced is not limited to four. In terms of the strength, the caulkedportions may be arranged at two or more positions (e.g. five to eightpositions) at regular intervals. Then, deformation of the cages in thecaulking process can be reduced.

Third Embodiment

[0109] This embodiment differs in structure from the second embodimentonly in the direction of the caulking. It is noted that the same orsimilar components or parts to those of the second embodiment aredenoted by like reference characters and detailed description thereof isnot repeated here.

[0110] Referring to FIGS. 8A-8D, thrust needle roller bearing 11′applied to a support structure which carries a thrust load of thetransmission has a plurality of needle rollers 2 and two annular cages13′ and 14′ that hold these needle rollers 2 at predetermined pitches inthe circumferential direction. Of two cages 13′ and 14′, upper cage 13′includes a roller holder portion 15 a which has its radially outerportion including, as shown in FIG. 8C, a sloping extension 13 a formedby bending the outer end of roller holder portion 15 a and an outerplate-like portion 13 b formed by bending the lower end of slopingextension 13 a in the radial direction. Roller holder portion 15 a ofupper cage 13 also has its radially inner portion including a slopingextension 13 c formed by bending the inner end of roller holder portion15 a and an inner plate-like portion 13 d′ formed by bending the lowerend of sloping extension 13 c in the radial direction.

[0111] Lower cage 14 includes a roller holder portion 16 a having itsradially outer portion including a sloping extension 14 a formed bybending the outer end of roller holder portion 16 a and an outerplate-like portion 14 b formed by bending the outer end of slopingextension 14 a in the radial direction. Roller holder portion 16 a oflower cage 14 also has its radially inner portion including a slopingextension 14 c formed by bending the inner end of roller holder portion16 a and an inner plate-like portion 14 d′ formed by bending the innerend of sloping extension 14 c in the radial direction.

[0112] As shown in FIG. 8C, outer plate-like portions 13 b and 14 b oftwo cages 13′ and 14′ are laid on each other so that they are in contactwith each other in the perpendicular direction, and the outermost partof outer plate-like portion 14 b is partially bent upward into C-shapeto partially caulk outer plate-like portion 14 b to outer plate-likeportion 13 b and thereby form partially caulked portions 17. On theother hand, inner plate-like portions 13 d′ and 14 d′ are laid on eachother so that they are in contact with each other in the perpendiculardirection and the innermost part of inner plate-like portion 14 d′ isbent upward.

[0113] Further, as shown in FIG. 8D, inner plate-like portions 13 d 40and 14 d′ are laid on each other so that they are in contact with eachother in the perpendicular direction and the innermost part of innerplate-like portion 14 d′ is partially bent upward into C-shape topartially caulk inner plate-like portion 14 d′ to inner plate-likeportion 13 d′ and thereby form partially caulked portions 18′. On theother hand, outer plate-like portions 13 b and 14 b are laid on eachother so that they are in contact with each other in the perpendiculardirection and the outermost part of outer plate-like portion 14 b isbent upward.

[0114] These partially caulked portions 17 and 18′ allow two cages 13′and 14′ to firmly be integrated on the inner and outer ends. Inaddition, the presence of partially caulked portions 17 and 18′ on thesame plane can further facilitate the caulking process, as compared withthe partial caulking of the above-described second embodiment.

Fourth Embodiment

[0115] This embodiment differs in structure from the above-discussedfirst to third embodiments only in the means for fixing the two cages.It is noted that the same or similar components or parts to those of thefirst to third embodiments are denoted by like reference characters anddescription thereof is not repeated here.

[0116] Referring to FIGS. 9A-9C, thrust needle roller bearing 21 appliedto a support structure which carries a thrust load of the transmissionhas a plurality of needle rollers 2 and two annular cages 23 and 24 thathold these needle rollers 2 at predetermined pitches in thecircumferential direction. Of two cages 23 and 24, upper cage 23 has aroller holder portion 25 a with its radially outer portion including asloping extension 23 a formed by bending the outer end of roller holderportion 25 a and an outer plate-like portion 23 b formed by bending thelower end of sloping extension 23 a in the radial direction, as shown inFIG. 9C. The radially inner portion of roller holder portion 25 a ofupper cage 23 has a sloping extension 23 c formed by bending the innerend of roller holder portion 25 a and an inner plate-like portion 23 dformed by bending the lower end of sloping extension 23 c in the radialdirection.

[0117] Lower cage 24 has its roller holder portion 26 a having aradially outer portion including a sloping extension 24 a formed-bybending the outer end of roller holder portion 26 a and an outerplate-like portion 24 b formed by bending the outer end of slopingextension 24 a in the radial direction. Roller holder portion 26 a oflower cage 24 also has its radially inner portion including a slopingextension 24 c formed by bending the inner end of roller holder portion26 a and an inner plate-like portion 24 d formed by bending the innerend of sloping extension 24 c in the radial direction.

[0118] As shown in FIG. 9C, outer plate-like portions 23 b and 24 b oftwo cages 23 and 24 are laid on each other so that they are in contactwith each other in the perpendicular direction and the outermost part ofouter plate-like portion 24 b is bent upward. On the other hand, innerplate-like portions 23 d and 24 d are laid on each other so that theyare in contact with each other in the perpendicular direction and theinnermost part of inner plate-like portion 24 d is bent upward. Theinnermost part of inner plate-like portion 23 d may be bent downward.According to this embodiment, the inner periphery and the outerperiphery of two cages 23 and 24 are integrally fixed at spot-weldedportions 27 and 28 as shown in FIG. 9A. These welded portions 27 and 28are arranged at four positions at regular intervals in thecircumferential direction with a phase difference of 45° between thewelded portions. Any deformation of the cages due to the welding is thusreduced. Here, the number of positions where the welding is done is notlimited to four. In terms of the strength, at least two (e.g. five toeight) spot-welded positions at regular intervals can reduce thedeformation of the cages due to the welding.

[0119] An alignment part 29 for alignment between two cages 23 and 24includes a projection 29 a formed on the outer rim of upper cage 23 anda notch 29 b formed in the outer rim of lower cage 24, and theseprojection 29 a and notch 29 b are engaged with each other in order toprevent displacement of phases of pockets 25 and 26 of two cages 23 and24. The structure of this alignment part 29 is not limited to theabove-described one. For example, so-called staking method may beapplied according to which the outer rim of lower cage 24 is partiallycaulked to be engaged with upper cage 23 and accordingly fixed, orengaging by a pin and a hole may be applied.

Fifth Embodiment

[0120] The first embodiment described above concerns the supportstructure which carries a thrust load in the torque converter portion ofa transmission. The thrust needle roller bearings of the first to fourthembodiments, however, may be employed as those carrying a thrust load ina gear mechanism of a transmission. Description is now given of thethrust needle roller bearing of the first embodiment that is applied toa support structure carrying a thrust load in a gear mechanism of atransmission.

[0121] Referring to FIGS. 10 and 11, a shaft serving as a main shaft 201is rotatably supported by bearings 203 located to front and back thereofin a case 202. Around shaft 201, a synchro hub 204 is provided and anidle gear 205 adjacent to one side of the hub is rotatably provided viaa rolling bearing 208. Idle gear 205 serving as a three-speed main gearhas a clutch gear 205 a between itself and synchro hub 204. On the rightof the three-speed main gear, namely idle gear 205, a radially-expandedportion 209 is formed around shaft 201. On the right of thisradially-expanded portion 209, another idle gear 207 is rotatablymounted on shaft 201 via a rolling bearing so that idle gear 207 is incontact with a right step surface 209 a. This idle gear 207 serves as atwo-speed main gear which is to be engaged/disengaged with/from anothersynchro hub.

[0122] A shaft serving as a counter shaft 211 placed in parallel toshaft 201 is rotatably supported in case 202 by a bearing 213 forexample. To this shaft 211, gears 215 and 217 engaging with idle gears205 and 207 on shaft 201 are fixed.

[0123] Between an end panel of idle gear 205 serving as the three-speedmain gear and a left step surface 209 b of radially-expanded portion 209of main shaft 201, a scissors gear (intervening gear) 218 is providedthat is the same in diameter as idle gear 205 and slightly different innumber of teeth from idle gear 205. This scissors gear 218 is rotatablymounted on shaft 201 so that this gear is in contact with the end panelof idle gear 205. These idle gear 205 and scissors gear 218 mesh withthe same gear 215 on counter shaft 211. Here, scissors gear 218 may beany which can mesh with the same gear 215 with which idle gear 205meshes. In this embodiment, the pitch circle, the addendum circle andthe dedendum circle of scissors gear 218 have respective diameters thatare the same as those of idle gear 205. Preferably, the number of teethof scissors gear 218 differs from that of idle gear 205 by at least one.Between scissors gear 218 and left step surface 209 b ofradially-expanded portion 209, a thrust needle roller bearing 1 servingas a support structure is provided.

[0124] This thrust needle roller bearing 1 has, as that of the firstembodiment, needle rollers 2, two cages 3 and 4 and races 220 and 221.In most cases, gear-contacting race 220 of thrust needle roller bearing1 that is in contact with scissors gear 218 is rotatable with respect toshaft 201 while race 221 contacting left step surface 209 b ofradially-expanded portion 209 is fixed to shaft 201 by means of a keyfor example.

[0125] Needle rollers 2 of thrust needle roller bearing 1 include needlerollers 2 a and 2 b arranged in two rows.

[0126] In a shift state, namely when synchro hub 204 engages with clutchgear 205 a of idle gear 205, shaft 201 and idle gear 205 rotate insynchronization with each other, while shaft 201 and scissors gear 218relatively rotate since the number of teeth of scissors gear 218 isslightly different from that of idle gear 205. Then, gear-contactingrace 220 and shaft-fixed race 221 of thrust needle roller bearing 1relatively rotate and accordingly needle rollers 2 make rotational andorbital motions.

[0127] Although the thrust needle roller bearing of the first embodimentis applied to the gear mechanism of the transmission in the descriptionabove, any of the thrust needle roller bearings of the second to fourthembodiment may be applied thereto.

[0128] According to the first to fifth embodiments described above,preferably needle rollers 2 a and 2 b have an end shape represented bysymbol “A” (round) defined by JIS (Japanese Industrial Standards) asshown in FIG. 12A, an end shape represented by symbol “F” (flat) definedby JIS as shown in FIG. 12B, or an end shape of a combination of thoserepresented respectively by symbols “A” and “F” defined by JIS.

[0129] In addition, although the embodiments are described in connectionwith the automatic transmission, the present invention is applicable tothrust support structures used widely for any transmissions of varioustypes. In particular, the present invention is applicable to the thrustsupport structure for use in an oil (lubricating oil) containing anadditive for the transmission.

[0130] Moreover, although the above-described support structures areattached between the turbine and the stator and between the stator andthe impeller, the present invention is not limited to this. The presentinvention is applicable to any portion of other transmissions thatcarries a thrust.

Sixth Embodiment

[0131] Referring to FIG. 13, a driving force generated by an engine (notshown) is transmitted from a crankshaft (not shown) to a continuouslyvariable transmission (CVT) mechanism 300, through a torque converter(not shown) and a forward/reverse switching mechanism 310.

[0132] Forward/reverse switching mechanism 310 has a planetary gearmechanism and multi-plate clutches 315 and 316. The planetary gearmechanism has a ring gear 313 a fixed to a shaft 301 a via a supportingmember 313, a sungear 301 b fixed to a primary shaft 301, and aplanetary pinion 312 a rotatably supported by a supporting member 312.This planetary pinion 312 a meshes with each of ring gear 313 a andsungear 301 b.

[0133] Multi-plate clutch 315 is incorporated, as a reverse brake,between the outer periphery of supporting member 312 and the innerperiphery of a housing 306. Multi-plate clutch 316 is included, as aforward clutch, between the outer periphery of primary shaft 301 and theinner periphery of supporting member 313. Multi-plate clutches 315 and316 have respective mechanisms capable of supplying an oil pressure.

[0134] When an oil pressure is supplied to allow multi-plate clutch(forward clutch) 316 to enter a connection state, rotations of shaft 301a are transmitted in the forward direction to primary shaft 301. When anoil pressure is supplied to allow multi-plate clutch (reverse brake) 315to enter a connection state, rotations of shaft 301 a are transmitted inthe reverse direction to primary shaft 301. In this way, the forward andbackward motions can be controlled.

[0135] CVT mechanism 300 has primary shaft 301 on the input side coupledwith forward/reverse switching mechanism 310, a primary pulley 302 ofthis primary shaft 301, a secondary shaft 303 on the output side and inparallel with primary shaft 301, a secondary pulley 304 of secondaryshaft 303, and a belt 305 looped over both of primary pulley 302 andsecondary pulley 304.

[0136] Primary pulley 302 has a fixed pulley 302 a fixed to primaryshaft 301, and a movable pulley 302 b mounted oppositely to the fixedpulley and sidably in the axial direction on primary shaft 301 with aball spline for example. Movable pulley 302 b thus slides in the axialdirection so that the distance between cone surfaces of the pulley,namely the groove width of the pulley, can be varied.

[0137] Secondary pulley 304 has a fixed pulley 304 a fixed to secondaryshaft 303 and a movable pulley 304 b mounted oppositely to the fixedpulley and sidably in the axial direction on secondary shaft 303 with aball spline for example. Movable pulley 304 b thus slides in the axialdirection so that the distance between cone surfaces of the pulley,namely the groove width of the pulley, can be varied.

[0138] Respective groove widths of both of the pulleys are varied tochange the diameter of contact between belt 305 and pulley 302 and thatbetween belt 305 and pulley 304. Accordingly, the ratio between thediameter of belt 305 wound around pulley 302 and the diameter of belt305 wound around pulley 304 changes. Rotations of primary shaft 301 arethus continuously varied in speed to be transmitted to secondary shaft303.

[0139] In this embodiment, thrust needle roller bearings are providedfor carrying thrust loads of shaft 301 a and primary shaft 301 on theinput side and secondary shaft 303 on the output side. The thrust needleroller bearings each have needle rollers arranged in two rows.

[0140] Referring to FIG. 14, thrust needle roller bearings 1 havingneedle rollers arranged in two rows is provided, for example, betweenthe inner ring of a rolling bearing 311 which rotatably supports primaryshaft 301 and supporting member 312, between supporting member 312 andsungear 301 b, between sungear 301 b and supporting member 313, andbetween supporting member 313 and housing 306, for example. Each thrustneedle roller bearing 1 has needle rollers 2 and two cages 3 and 4 forholding needle rollers 2. Needle rollers 2 include needle rollers 2 aand 2 b arranged in two rows.

[0141] This thrust needle roller bearing 1 has its structure asspecifically described below.

[0142] Referring to FIGS. 2A-2E and FIG. 3, thrust needle roller bearing1 has a plurality of needle rollers 2 and two annular cages 3 and 4 thathold these needle rollers 2 at predetermined pitches in thecircumferential direction. Two cages 3 and 4 respectively have aplurality of rectangular pockets 5 and 6 with the length in the radialdirection larger than the length L of needle rollers 2, and are formedby pressing of a steel plate which is for example a cold-rolled steelplate (SPCC). Roller holder portions 5 a and 6 a are formed respectivelyon opposite sides of pockets 5 and 6 to extend toward and face eachother. Needle rollers 2 are thus held by roller holder portions 5 a and6 a therebetween in the direction from above and below. Instead of cages3 and 4 as described above, any cages produced through draw-forming of aband steel by means of a press or through shaving may be used.

[0143] Needle rollers 2 include needle rollers 2 a on the radially outerside and needle rollers 2 b on the radially inner side, and are arrangedin two rows in pockets 5 and 6. The double-row arrangement reduces adifference in circumferential velocity of revolution between theradially outer part and the radially inner part, and thereby thedifferential slip between the rollers and the raceway surface (notshown) is reduced. Then, less heat is generated from the contact portionand the surface damage (smearing) can be prevented.

[0144] Here, respective lengths L1 and L2 of double-row needle rollers 2a and 2 b are equal to each other. These lengths, however, may beselected to satisfy a relation, L2 (length of the radially innerroller)≦L1 (length of the radially outer roller) or L1≦L2 depending onconditions in use. Preferably, the length of radially outer needleroller 2 a is longer, for example, 1.2 times as long as the length ofradially inner needle roller 2 b so that the load capacity on theradially outer part is increased.

[0145] As shown in FIG. 2D, the length La in the radial direction ofroller holder portions 5 a and 6 a can be made smaller than the length Lof needle rollers so that a lubricating oil can readily pass betweenneedle rollers 2 and cages 3 and 4 through depressions 5 b and 6 bformed on both ends of roller holder portions 5 a and 6 a.

[0146] Further, as shown in FIG. 2E, respective corners (F) of rollerholder portions 5 a and 6 a may be rounded when pockets 5 and 6 arepunched by a press, or may be rounded to become smooth throughsurface-pressing of the inner rim after the punching. In this way,without breaking a lubricating-oil film formed on the surfaces of needlerollers 2 a and 2 b, needle rollers 2 a and 2 b can stably be guided andheld. Here, the corners are “rounded” so that the sharp edges of thecorners are removed to become round.

[0147] Of two cages 3 and 4, upper cage 3 has its roller holder portion5 a with its radially outer portion including a sloping extension 3 aformed by bending the outer end of roller holder portion 5 a and anouter plate-like portion 3 b formed by bending the lower end of slopingextension 3 a in the radial direction. Roller holder portion 5 a of cage3 also has its radially inner portion including a sloping extension 3 cformed by bending the inner end of roller holder portion 5 a and aninner plate-like portion 3 d formed by bending the lower end of slopingextension 3 c in the radial direction.

[0148] Lower cage 4 with the pockets punched with the same die as thatfor upper cage 3 has roller holder portion 6 a with its radially outerportion including a sloping extension 4 a formed by bending the outerend of roller holder portion 6 a and an outer plate-like portion 4 bformed by bending the outer end of sloping extension 4 a. Roller holderportion 6 a of lower cage 4 has its radially inner portion including asloping extension 4 c formed by bending the inner end of roller holderportion 6 a and an inner plate-like portion 4 d formed by bending theinner end of sloping extension 4 c.

[0149] Respective outer plate-like portions 3 b and 4 b of two cages 3and 4 are laid on each other so that these plates are in contact witheach other in the direction perpendicular to the radial direction asseen in FIG. 2C (herein referred to as perpendicular direction). Theoutermost part of outer plate-like portion 4 b is bent upward intoC-shape so that outer plate-like portion 4 b is caulked to outerplate-like portion 3 b to form a caulked portion 7. Inner plate-likeportions 3 d and 4 d are also laid on each other so that these platesare in contact with each other in the perpendicular direction. Theinnermost part of inner plate-like portion 3 d is bent downward intoC-shape so that inner plate-like portion 3 d is caulked to innerplate-like portion 4 d to form a caulked portion 8. Caulked portions 7and 8 are provided along the entire outermost and innermost edges of twocages 3 and 4.

[0150] Two cages 3 and 4 are thus firmly integrated into a unit bycaulking of the outermost and innermost edges with caulked portions 7and 8. Therefore, two cages 3 and 4 are never separated from each otherduring a drive. Moreover, a large area of contact between the endsurfaces of needle rollers 2 and pockets 5 and 6 is secured in regionsof outer plate-like portions 3 b and 4 b and inner plate-like portions 3d and 4 d so that drilling wear can be reduced.

[0151] When two cages 3 and 4 are fixed, a thickness T₁, in theperpendicular direction, of the radially outer part of the cages and athickness T₂, in the perpendicular direction, of the radially inner partthereof are smaller than a thickness To in the perpendicular directionof roller holder portions 5 a and 6 a of two cages 3 and 4, because ofthe presence of sloping extensions 3 a, 4 a, 3 c and 4 c. In otherwords, between caulked portions 7 and 8 and corresponding ends of needlerollers 2, there are provided flat portions (outer plate-like portions 3b and 4 b and inner plate-like portions 3 d and 4 d) having across-sectional height smaller than that of roller holder portions 5 aand 6 a.

[0152] The path of the lubricating oil is described below in connectionwith, for example, thrust needle roller bearing 1 having theabove-described structure and placed between supporting member 313 andhousing 306 shown in FIGS. 13 and 14.

[0153] Referring to FIG. 15, thrust needle roller bearing 1 is clearancefit by using caulked portion 8 of upper cage 3 as a guide surface, sothat needle rollers 2 roll between supporting member 313 and housing306. As supporting member 313 rotates, cages 3 and 4 accordingly rotatewith supporting member 313 so that needle rollers 2 roll between racewaysurface 10 a of supporting member 313 and raceway surface 9 a of housing306. Here, a lubricating oil is supplied from a hydraulic source (notshown) through an oil path into thrust needle roller bearing 1.

[0154] The lubricating oil is supplied from the radially inner side ofthrust needle roller bearing 1 as indicated by arrow b, thereafterpassed through a space formed by the outer surfaces of needle rollers 2and cages 3 and 4 as indicated by arrow c, lubricates the portionsbetween the side surfaces of needle rollers 2 and roller holder portions5 a and 6 a of cages 3 and 4, between end surfaces of needle rollers 2and cages 3 and 4, and between the side surfaces of needle rollers 2 andraceway surfaces 9 a and 10 a, and is then discharged as indicated byarrow d through the portion between raceway surface 10 a and theradially outer portion of cage 4 with respect to roller holder portion 6a and through the portion between raceway surface 9 a and the radiallyouter portion of cage 3 with respect to roller holder portion 5 a.

[0155] Regarding the lubrication of the above-described portions by thelubricating oil, since the perpendicular thicknesses T₁ and T₂ of theradially outer and inner portions of roller holder portions 5 a and 6 aof two cages 3 and 4 are made smaller than the perpendicular thicknessT₀ of roller holder portions 5 a and 6 a (see FIG. 2C) and accordinglythe cross-sectional area of the space formed between raceway surface 10a and the radially inner portion of lower cage 4 with respect to rollerholder portion 6 a is larger than conventional ones, not only theoutflow of the lubricating oil but also the inflow of the lubricatingoil is enhanced. Seizure of any part of the bearing can surely beprevented and drilling wear of the end surfaces of needle rollers 2 andpockets 5 and 6 of cages 3 and 4 can be reduced. Moreover, since thelubricating oil is less prone to be hindered from passing by cages 3 and4, the lubricating oil hardly stays. Then, any increase of the oiltemperature can be reduced, the strength of the cages is increased andthe durability of the bearing can further be improved.

[0156] A procedure for manufacturing the above-described thrust needleroller bearing 1 is detailed below.

[0157] Referring to FIG. 5, two cages 3 and 4 and needle rollers 2 thathave been quenched and tempered or have not been quenched are set.Specifically, the cages and needle rollers are assembled in such amanner that roller holder portions 5 a and 6 a formed respectively atpockets 5 and 6 of two cages 3 and 4 hold needle rollers 2 therebetweenin the direction from above and below (step S1 a). Then, the outermostpart of outer plate-like portion 4 b is bent upward into C-shape to formcaulked portion 7 while the innermost part of inner plate-like portion 3d is bent downward into C-shape to form caulked portion 8, andaccordingly two cages 3 and 4 are integrated into a unit and fixed (stepS2 a). Here, needle rollers 2 are made of, for example, a SUJ bearingsteel which is a high carbon chromium bearing steel of type 1 or type 2,and the steel is oil-quenched at a temperature of 840° C. for 30 minutesand then tempered at a temperature of 180° C. for 90 minutes to adjustthe surface hardness to approximately 700-750 by Vickers hardness (Hv).

[0158] After this, with needle rollers 2 and two cages 3 and 4 being setas described above, they are heat-treated (e.g. carburized, quenched andtempered or quenched and tempered after carbonitrided) (step S3 a). Athrust needle roller bearing is thus produced. In this case, thecarburization is done, for example, by carburization for example at atemperature of 850° C. for 35 minutes (in RX gas atmosphere), quenchingin oil, and then tempering at a temperature of 165° C. for 60 minutes.The carbonitriding is done, for example, by carbonitriding in acarbonitriding atmosphere (1-3% by volume of ammonia is added to the RXgas) at a temperature of 840-850° C. for 35 minutes and subsequent quickcooling in oil or gas.

[0159] Here, two cages 3 and 4 may be improved in strength by beingsoft-nitrided in advance at a temperature of 570-580° C. for 35 minutes.No preliminary heat-treatment may be performed on needle rollers 2.However, if needle rollers 2 are subjected to heat treatment or throughquenching in advance before being assembled, an advantage is obtainedthat the strength is further improved by the subsequent carburizing orcarbonitriding while a disadvantage of an increase of the manufacturingsteps is accompanied. At any rate, as compared with conventionallyemployed processes in which two cages 3 and 4 and needle rollers 2 areseparately heat-treated and the caulked portions are annealed, themanufacturing process is simplified.

[0160] If the soft-nitriding of cages 3 and 4 is done, the process shownin FIG. 6 may be used. Namely, cages 3 and 4 and needle rollers 2 areseparately heat-treated (e.g. carburized, quenched and tempered orcarbonitrided and then quenched and tempered) (step S1 b), cages 3 and 4and needle rollers 2 are set (step S2 b) and thereafter cages 3 and 4are caulked (step S3 b).

[0161] Thrust needle roller bearing 1 which is manufactured through theabove-describe procedure can offer specific characteristics that aredescribed in detail below.

[0162] With regard to needle rollers 2, the surface layer thereof is acarburized or carbonitrided layer and thus the hardness of the surfacelayer is higher than that of conventional ones. Therefore, even ifhigh-hardness foreign matters are caught in rollers, indentations arehardly made, which contributes to extension of the lifetime. In thecarbonitriding process, a nitrogen-rich layer is formed and such a largeamount of retained austenite as 20% by volume can be obtained. Thisfeature contributes to an increase of the hardness as well as anextension of the lifetime since the large amount of retained austeniteplastically deforms to alleviate stress concentration, which occurs inconventional bearings when a high-hardness foreign matter is caughtbetween raceway surfaces 9 a and 10 a that leaves an indentationresulting in a source of stress concentration around the indentation.Here, the nitrogen-rich layer may specifically be at least 0.1 mm inthickness and at least 750 Hv in surface hardness. Moreover, theinternal hardness can be increased to be almost equal to the surfacehardness so that the strength of the entire needle rollers can beimproved. Accordingly, when used under harsh conditions, for example,heavy load, the product of the present invention can sufficiently endurethe load and satisfy a desired lifetime.

[0163] With regard to cages 3 and 4, as needle rollers 2, the surfacelayer of the cages is a carburized layer or carbonitrided layer, and asurface hardness of at least 400 Hv, or at least 600 Hv can be achieveddepending on conditions of heat-treatment. The wear resistance can thusbe enhanced as compared with conventional products.

Seventh Embodiment

[0164] This embodiment has its structure different from that of thesixth embodiment only in the shape of cages and caulking method. It isnoted that the same or similar components or parts to those of the sixthembodiment are denoted by like reference characters and descriptionthereof is not repeated here.

[0165] Referring to FIGS. 7A-7E, thrust needle roller bearing 11 appliedto a support structure for the continuously variable transmission has aplurality of needle rollers 2 and two annular cages 13 and 14 that holdthese needle rollers 2 at predetermined pitches in the circumferentialdirection. As shown in FIG. 7D, the upper one of two cages 13 and 14,namely upper cage 13, includes a roller holder portion 15 a having itsradially outer portion which includes a sloping extension 13 a formed bybending the outer end of roller holder portion 15 a and an outerplate-like portion 13 b formed by bending the lower end of slopingextension 13 a in the radial direction.

[0166] Roller holder portion 15 a of upper cage 13 also has its radiallyinner portion which includes a sloping extension 13 c formed by bendingthe inner end of roller holder portion 15 a and an inner plate-likeportion 13 d formed by bending the lower end of sloping extension 13 cin the radial direction.

[0167] Lower cage 14 with pockets punched with the same die as that forupper cage 13 has roller holder portion 16 a with its radially outerportion including a sloping extension 14 a formed by bending the outerend of roller holder portion 16 a and an outer plate-like portion 14 bformed by bending the outer end of sloping extension 14 a in the radialdirection. Roller holder portion 16 a of lower cage 14 has its radiallyinner portion including a sloping extension 14 c formed by bending theinner end of roller holder portion 16 a and an inner plate-like portion14 d formed by bending the inner end of sloping extension 4 c in theradial direction.

[0168] As shown in FIG. 7D, respective outer plate-like portions 13 band 14 b of two cages 13 and 14 are laid on each other so that theseplates are in contact with each other in the direction perpendicular tothe radial direction (herein perpendicular direction). The outermostpart of outer plate-like portion 14 b is partially bent upward intoC-shape so that outer plate-like portion 14 b is caulked partially toouter plate-like portion 13 b to form partially caulked portions 17.Inner plate-like portions 13 d and 14 d are also laid on each other sothat these plates are in contact with each other in the perpendiculardirection, and the innermost part of inner plate-like portion 13 d isbent downward.

[0169] Further, as shown in FIG. 7E, outer plate-like portions 13 b and14 b are laid on each other so that they are in contact with each otherin the perpendicular direction and the outermost part of outerplate-like portion 14 b is bent upward. Inner plate-like portions 13 dand 14 d are laid on each other so that they are in contact with eachother in the perpendicular direction and the innermost part of innerplate-like portion 13 d is partially bent downward into C-shape so thatinner plate-like portion 13 d is partially caulked to inner plate-likeportion 14 d to form partially caulked portions 18. With these partiallycaulked portions 17 and 18, the inner and outer edges of two cages 13and 14 are firmly integrated and, in this case, the manufacturingprocess can remarkably be simplified as compared with the caulking ofthe entire periphery of the thrust needle roller bearing (FIGS. 2A-2E)of the sixth embodiment described above.

[0170] An alignment part 19 for alignment between two cages 13 and 14includes, for example, a notch (not shown) made in the outer rim ofupper cage 13 and a projection (not shown) formed on the outer rim oflower cage 14, and the projection and the notch are engaged with eachother so as to prevent displacement of pitches of pockets 15 and 16 ofcages 13 and 14 in the caulking process for example. Those partiallycaulked portions 17 and 18 are arranged at four positions on theperiphery with a phase difference of 45° between the caulked portions.In this way, deformation of the cages in the caulking process isreduced. Here, the number of positions where the caulked portions areplaced is not limited to four. In terms of the strength, the caulkedportions may be arranged at two or more positions (e.g. five to eightpositions) at regular intervals. Then, deformation of the cages in thecaulking process can be reduced.

Eighth Embodiment

[0171] This embodiment differs in structure from the seventh embodimentonly in the direction of the caulking. It is noted that the same orsimilar components or parts to those of the seventh embodiment aredenoted by like reference characters and detailed description thereof isnot repeated here.

[0172] Referring to FIGS. 8A-8D, thrust needle roller bearing 11′applied to a support structure for the continuously variabletransmission has a plurality of needle rollers 2 and two annular cages13′ and 14′ that hold these needle rollers 2 at predetermined pitches inthe circumferential direction. Of two cages 13′ and 14′, upper cage 13′includes a roller holder portion 15 a which has its radially outerportion including, as shown in FIG. 8C, a sloping extension 13 a formedby bending the outer end of roller holder portion 15 a and an outerplate-like portion 13 b formed by bending the lower end of slopingextension 13 a in the radial direction. Roller holder portion 15 a ofupper cage 13 also has its radially inner portion including a slopingextension 13 c formed by bending the inner end of roller holder portion15 a and an inner plate-like portion 13d′ formed by bending the lowerend of sloping extension 13 c in the radial direction.

[0173] Lower cage 14 includes a roller holder portion 16 a having itsradially outer portion including a sloping extension 14 a formed bybending the outer end of roller holder portion 16 a and an outerplate-like portion 14 b formed by bending the outer end of slopingextension 14 a in the radial direction. Roller holder portion 16 a oflower cage 14 also has its radially inner portion including a slopingextension 14 c formed by bending the inner end of roller holder portion16 a and an inner plate-like portion 14 d′ formed by bending the innerend of sloping extension 14 c in the radial direction.

[0174] As shown in FIG. 8C, outer plate-like portions 13 b and 14 b oftwo cages 13′ and 14′ are laid on each other so that they are in contactwith each other in the perpendicular direction, and the outermost partof outer plate-like portion 14 b is partially bent upward into C-shapeto partially caulk outer plate-like portion 14 b to outer plate-likeportion 13 b and thereby form partially caulked portions 17. On theother hand, inner plate-like portions 13 d′ and 14 d′ are laid on eachother so that they are in contact with each other in the perpendiculardirection and the innermost part of inner plate-like portion 14 d′ isbent upward.

[0175] Further, as shown in FIG. 8D, inner plate-like portions 13 d′ and14 d′ are laid on each other so that they are in contact with each otherin the perpendicular direction and the innermost part of innerplate-like portion 14 d′ is partially bent upward into C-shape topartially caulk inner plate-like portion 14 d′ to inner plate-likeportion 13 d′ and thereby form partially caulked portions 18′. On theother hand, outer plate-like portions 13 b and 14 b are laid on eachother so that they are in contact with each other in the perpendiculardirection and the outermost part of outer plate-like portion 14 b isbent upward.

[0176] These partially caulked portions 17 and 18′ allow two cages 13′and 14′ to firmly be integrated on the inner and outer ends. Inaddition, the presence of partially caulked portions 17 and 18′ on thesame plane can further facilitate the caulking process, as compared withthe partial caulking of the above-described second embodiment.

Ninth Embodiment

[0177] This embodiment differs in structure from the above-discussedsixth to eighth embodiments only in the means for fixing the two cages.It is noted that the same or similar components or parts to those of thesixth to eighth embodiments are denoted by like reference characters anddescription thereof is not repeated here.

[0178] Referring to FIGS. 9A-9C, thrust needle roller bearing 21 appliedto a support structure which carries a thrust load of the transmissionhas a plurality of needle rollers 2 and two annular cages 23 and 24 thathold these needle rollers 2 at predetermined pitches in thecircumferential direction. Of two cages 23 and 24, upper cage 23 has aroller holder portion 25 a with its radially outer portion including asloping extension 23 a formed by bending the outer end of roller holderportion 25 a and an outer plate-like portion 23 b formed by bending thelower end of sloping extension 23 a in the radial direction, as shown inFIG. 9C. The radially inner portion of roller holder portion 25 a ofupper cage 23 has a sloping extension 23 c formed by bending the innerend of roller holder portion 25 a and an inner plate-like portion 23 dformed by bending the lower end of sloping extension 23 c in the radialdirection.

[0179] Lower cage 24 has its roller holder portion 26 a having aradially outer portion including a sloping extension 24 a formed bybending the outer end of roller holder portion 26 a and an outerplate-like portion 24 b formed by bending the outer end of slopingextension 24 a in the radial direction. Roller holder portion 26 a oflower cage 24 also has its radially inner portion including a slopingextension 24 c formed by bending the inner end of roller holder portion26 a and an inner plate-like portion 24 d formed by bending the innerend of sloping extension 24 c in the radial direction.

[0180] As shown in FIG. 9C, outer plate-like portions 23 b and 24 b oftwo cages 23 and 24 are laid on each other so that they are in contactwith each other in the perpendicular direction and the outermost part ofouter plate-like portion 24 b is bent upward. On the other hand, innerplate-like portions 23 d and 24 d are laid on each other so that theyare in contact with each other in the perpendicular direction and theinnermost part of inner plate-like portion 24 d is bent upward. Theinnermost part of inner plate-like portion 23 d may be bent downward.According to this embodiment, the inner periphery and the outerperiphery of two cages 23 and 24 are integrally fixed at spot-weldedportions 27 and 28 as shown in FIG. 9A. These welded portions 27 and 28are arranged at four positions at regular intervals in thecircumferential direction with a phase difference of 45° between thewelded portions. Any deformation of the cages due to the welding is thusreduced. Here, the number of positions where the welding is done is notlimited to four. In terms of the strength, at least two (e.g. five toeight) spot-welded positions at regular intervals can reduce thedeformation of the cages due to the welding.

[0181] An alignment part 29 for alignment between two cages 23 and 24includes a projection 29 a formed on the outer rim of upper cage 23 anda notch 29 b formed in the outer rim of lower cage 24, and theseprojection 29 a and notch 29 b are engaged with each other in order toprevent displacement of phases of pockets 25 and 26 of two cages 23 and24. The structure of this alignment part 29 is not limited to theabove-described one. For example, so-called staking method may beapplied according to which the outer rim of lower cage 24 is partiallycaulked to be engaged with upper cage 2.

[0182] According to the description above, although the outermost partor the innermost part of cage 3 or 4 is partially or entirely bent alongthe periphery into C-shape to be caulked as shown in FIG. 16A, the shapeof the bent and caulked portion is not limited to this and may be any ofrespective shapes shown in FIGS. 16B-16E.

[0183] Further, although the thrust needle roller bearing of the sixthembodiment is applied to the CVT, any of the thrust needle rollerbearings of the seventh to ninth embodiments may be applied to the CVT.

[0184] According to the sixth to ninth embodiments described above,preferably needle rollers 2 a and 2 b have an end shape represented bysymbol “A” (round) defined by JIS (Japanese Industrial Standards) asshown in FIG. 12A, an end shape represented by symbol “F” (flat) definedby JIS as shown in FIG. 12B, or an end shape of a combination of thoserepresented respectively by symbols “A” and “F” defined by JIS.

[0185] Moreover, although the structures of the above-describedembodiments carry the thrust loads of the input shafts (shaft 301 a,primary shaft 301), the thrust needle roller bearing having needlerollers arranged in two rows may be applied to a support structurecarrying the thrust load of the output shaft (secondary shaft 303).

[0186] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

What is claimed is:
 1. A support structure carrying a thrust load of atransmission having an input shaft with its rotation changed stepwise orcontinuously to be transmitted to an output shaft, wherein a thrustneedle roller bearing carries a thrust load generated by rotation ofsaid input shaft or said output shaft and said thrust needle rollerbearing has needle rollers arranged in two rows.
 2. The supportstructure according to claim 1, wherein said transmission includes atorque converter having an impeller and a turbine opposite to each otherwith a stator therebetween, and said thrust needle roller bearing havingsaid needle rollers arranged in two rows is provided at, at least one ofplaces respectively between said stator and said impeller and betweensaid stator and said turbine.
 3. The support structure according toclaim 1, wherein said transmission includes a gear mechanism constitutedof a plurality of gears, and said thrust needle roller bearing havingsaid needle rollers arranged in two rows is provided to carry a thrustload generated by rotation of said gears.
 4. The support structureaccording to claim 1, wherein said support structure carrying a thrustload of said transmission is a support structure for a continuouslyvariable transmission having an input shaft with its rotation changedcontinuously to be transmitted to an output shaft.
 5. The supportstructure according to claim 4, wherein the width of a groove of a firstpulley provided on said input shaft and the width of a groove of asecond pulley provided on said output shaft are each changed to vary thediameter of contact of a belt, looped over said first pulley and saidsecond pulley, with said first pulley as well as the diameter of contactof said belt with said second pulley and thereby continuously changerotation of said input shaft to transmit resultant rotation to saidoutput shaft.
 6. The support structure according to claim 1, whereinsaid thrust needle roller bearing has a plurality of needle rollers andtwo annular cages, said two cages respectively have a plurality ofpockets having a radial length slightly larger than that of said needlerollers, roller holder portions formed at said plurality of pocketsrespectively are structured to allow said two cages to hold said needlerollers therebetween in the direction from above and below, and needlerollers are arranged in two rows in each of said plurality of pockets.7. The support structure according to claim 6, wherein one of said twocages is caulked to be fixed to the other cage on at least one of theradially outermost end and the radially innermost end of said two cages.8. The support structure according to claim 6, wherein said two cagesare caulked on both of the radially outermost end and the radiallyinnermost end of said two cages, and a flat portion having across-sectional height lower than that of said roller holders isprovided between a caulked portion produced by caulking of said twocages and corresponding ends of said needle rollers.
 9. The supportstructure according to claim 6, wherein said two cages are welded on atleast one of the radially innermost end and the radially outermost endof said two cages.
 10. The support structure according to claim 1,wherein said needle rollers have an end-surface shape represented bysymbol A, symbol F or a combination of those represented by symbols Aand F defined by JIS.
 11. The support structure according to claim 1,wherein one of said needle rollers arranged in two rows that is locatedon the radially outer side has its length which is at least the lengthof the needle roller located on the radially inner side.
 12. The supportstructure according to claim 1, wherein said roller holder portions havetheir corners rounded smoothly.
 13. A thrust needle roller bearing usedfor a support structure carrying a thrust load of the transmission asrecited in claim
 1. 14. A method of manufacturing the support structurecarrying a thrust load of the transmission as recited in claim 1,wherein said thrust needle roller bearing has a plurality of needlerollers and two annular cages, said two cages respectively have aplurality of pockets having a radial length slightly larger than that ofsaid needle rollers, roller holder portions formed at said plurality ofpockets respectively are structured to allow said two cages to hold saidneedle rollers therebetween in the direction from above and below, andsaid needle rollers are arranged in two rows in each of said pluralityof pockets, and said two cages and said needle rollers are assembled sothat said cages hold said needle rollers therebetween in the directionfrom above and below by said roller holder portions formed respectivelyat said plurality of pockets, one of said two cages is caulked to theother cage, and thereafter said two cages and said needle rollers arecarburized, quenched and tempered.
 15. A method of manufacturing thesupport structure carrying a thrust load of the transmission as recitedin claim 1, wherein said thrust needle roller bearing has a plurality ofneedle rollers and two annular cages, said two cages respectively have aplurality of pockets having a radial length slightly larger than that ofsaid needle rollers, roller holder portions formed at said plurality ofpockets respectively are structured to allow said two cages to hold saidneedle rollers therebetween in the direction from above and below, andsaid needle rollers are arranged in two rows in each of said pluralityof pockets, and said two cages and said needle rollers are eachcarburized, quenched and tempered and thereafter assembled so that saidcages hold said needle rollers therebetween in the direction from aboveand below by said roller holder portions formed respectively at saidplurality of pockets, and one of said two cages is caulked to the othercage.